SO2 SCRUBBING BY A SHALLOW AQUIFER: A GROUNDWATER MODEL FOR MOUNT ST. HELENS

SO₂ released from magma intruding into the shallow subsurface may react with groundwater, a process that “scrubs” volcanic emissions of SO₂. Gas-emission measurements during eruptive activity at Mount St. Helens in fall 2004 have been interpreted as indicative of SO₂ scrubbing by groundwater during the first ~10 days of volcanic unrest. An intriguing possibility is that a shallow aquifer beneath the crater glacier may have played a key role in scrubbing. The presence of such an aquifer, consisting primarily of coarse (mostly avalanche) deposits that accumulated in the crater following the 1980 eruptions, has been inferred on glaciological grounds (J.S. Walder and others, 2007, Ann. Glaciol. 45, 14-20). The potential for SO₂ scrubbing by the shallow subglacial aquifer at Mount St. Helens was assessed by using a finite-volume groundwater model. The model couples mass- and energy transport and considers the effect of introducing localized sources of magmatic gases. Initially, ambient flow within the aquifer, which is driven by surface-derived waters, is strongly perturbed as magmatic H₂O, the dominant gas species, condenses within the aquifer. Once the aquifer above the magmatic gas source locally reaches the boiling point, the enthalpy flux of the gas drives vaporization. For a given aquifer permeability, the enthalpy flux may be large enough for the vaporization rate locally to exceed the ability of the aquifer to “refill”: that is, the aquifer locally dries out. This would be a condition under which SO₂ could then escape. Model calculations using plausible values of gas emission rates (including H₂O) and aquifer permeability suggest the onset of localized aquifer drying after a period of days to weeks, generally consistent with observations. This model should also be pertinent to other volcanoes with crater glaciers.